1. Field of the Invention
The present invention relates to a printing apparatus for printing an image on a printing medium such as a printing paper sheet and a printing medium conveyance control method of the apparatus and, particularly to, for example, a printing apparatus for performing color printing based on an inkjet method and a printing medium conveyance control method.
2. Description of the Related Art
Inkjet printing apparatuses (to be referred to as printing apparatuses hereinafter) have proliferated in wide industrial fields as relatively simple and excellent printing means, and are required to increase the printing speed and print images of higher quality. Conveyance control of a printing medium such as a printing paper sheet in the printing apparatus has very large influence on image quality. If this control is not correctly done, for example, the landing position of ink discharged to the printing medium shifts. This shift leads to stripes and considerably lowers the image quality.
For positioning in conventional printing medium conveyance control, for example, a digital signal converted based on the zero-cross point of a sinusoidal analog signal output from an encoder is used. In conveyance control using such a digital signal, however, it is feared that control especially at the time of conveyance stop could not meet required accuracy.
To meet requirements of higher image quality, a method as described in Japanese Patent Laid-Open No. 8-201111 has been proposed. More specifically, a sinusoidal analog signal output from an encoder is converted into a digital signal by an A/D converter. The digital signal is supplied to a LPF (Low-Pass Filter) to filter out variations in high frequency components in the signal, that is, lower bits, and then supplied to an interpolation circuit.
In the method described in Japanese Patent Laid-Open No. 8-201111, however, when performing printing medium conveyance control at a relatively high speed, that is, when the rate of change in the analog signal is larger than its sampling period, data that is moving-averaged in the LPF largely varies. For this reason, the correctness of data output from the LPF lowers. On the other hand, if the population parameter of data to be moving-averaged is decreased to solve the above problem, noise components included in the data can hardly be filtered out. In this case as well, the correctness of data output from the LPF lowers. It is therefore impossible to implement accurate positioning in control at the time of printing medium conveyance stop.